1. Field of the Invention
The present invention relates to an anti-fogging coating which can prevent fogging of an optical substrate caused by moisture, and optical parts obtained by applying the anti-fogging coating to optical substrates such as lenses, prisms and mirrors.
2. Related Background Art
The reason why various kinds of optical substrates typified by lenses and mirrors in optical instruments fog up is that moisture in the air adheres thereto in the form of fine waterdrops because their surface humidity is lowered to a dew point or lower, and so light is diffusely reflected. Accordingly, it is considered that the fogging can be prevented by preventing the formation of waterdrops on the surface of the substrate. As such anti-fogging methods, for example, the following four factors have come to be considered. (A) Adjustment of wetting; (B) impartment of water-absorbing property; (C) impartment of water repellency; and (D) adjustment of humidity by heating.
Although various methods have been attempted as yet, all the attempts are insufficient. It is now practiced to adjust humidity of the surface of an optical substrate by heating, thereby producing an anti-fogging effect on lenses of copying machines, rear windshields for automobiles and high-grade dressing tables. However, such a method involves many problems. For example, since a power source is required, its application fields are limited, and accidents such as fires may happen.
In order to offset the above demerits, there have been developed films formed of an anti-fogging coating composition comprising a high water-resistant organic polymer containing a surfactant. This anti-fogging coating composition is so designed that a film made hydrophilic by the surfactant and polyether polyol absorbs moisture to exhibit anti-fogging property, and in the case where the moisture exceeds the critical point of water absorption of this film, wetting is adjusted by the surfactant contained to maintain good transparency. However, since the surfactant easily dissolves in water to run out, the anti-fogging properties and strength of the film are markedly lowered.
There have been further proposed anti-fogging coating films formed mainly of an organic substance having polyvinyl alcohol or a polymer of hydroxyethyl methacrylate as a main component (for example, Japanese Patent Application Laid-Open No. 55-110201). However, such a film has low hardness and abrasion resistance, and poor weathering resistance.
As known water-absorbing substances, there are natural polymers including starch type polymers such as hydrolyzates of starch-acrylonitrile graft polymers, and cellulosic polymers such as cellulose-acrylonitrile graft polymers; and synthetic polymers including polyvinyl alcohol type polymers such as crosslinked polyvinyl alcohol, acrylic polymers such as crosslinked sodium polyacrylate, and polyether type polymers such as crosslinked polyethylene glycol diacrylate.
When one of these substances is coated and formed into a film for use as an anti-fogging film for the purpose of preventing fogging, the lastingness of its effect is markedly improved compared with the case of the surfactant. When the water-absorbing substance is used for the anti-fogging film, however, the anti-fogging properties thereof tends to be impaired when an anti-reflection layer formed of a substance having a low refractive index or a protective layer is coated thereon.
When the water-absorbing substance is made into a thin film and its optical thickness [(geometrical thickness, d).times.(refractive index, n)] is adjusted to an odd number times a quarter of the target wavelength whose reflection is to be prevented, thus using it as an anti-reflection film, the film tends not to attain sufficient anti-fogging properties because the thickness of the water-absorbing film is too thin.